Diagnosis of the productivity gap using a crop model. Methodology and case study of small-scale maize production in central Brazil

Reducing the local gap between potential and actual yields requires first diagnosing its causes. This article proposes a method for this purpose, based on a survey in farmers' fields and on the building and use of an ad hoc crop model. Applied to maize of small-scale farms in central Brazil, and using the STICS model as a starting point, this method allowed the identification of the constraints responsible for the local gap and to assess the specific impact of each constraint and of the main interactions between constraints on yield. STICS was modified in order (i) to improve its water balance module for the tropical context, and (ii) to account for Aluminum toxicity, water excess and competition between crop and weeds for light, water and nitrogen. The main technical weaknesses of the cropping systems were inappropriate sowing dates, N-fertilization and weeding sequences, as well as delays between tillage and sowing, favoring weed growth.